1. Field of the Invention
The present invention is directed generally to a circuit for operating a group of relays which are driven by a common drive voltage.
2. Description of the Related Art
In electromagnetic relays which have a winding for attracting an armature, there is a general problem in that the windings require a relatively high pick-up excitation signal, particularly given temperature fluctuations and given fluctuations in the supply voltage. Once the armature of the relay is attracted, a significantly lower holding excitation signal is adequate to maintain the relay in the "on" condition. Thus, if the high pick-up voltage also termed pick-up voltage is applied during the continuous operation of the relay, an unnecessarily high amount of energy is consumed and the relay is undesirably heated.
To drive individual relays, it has long been known to lower the pick-up voltage to a lesser holding voltage after the attraction of the armature. This may be accomplished, for example, through the use of a mere time control which switches to the holding voltage after a pick-up pulse of a defined duration. Instead, the successful attraction of the armature can be positively identified and, thus, the switch to the lower holding voltage may be undertaken only after the armature is successfully attracted. According to German Patent Document DE 36 15 908 A1, a decrease in the current at the moment the magnetic circuit of the armature closes is identified and evaluated for switching to the lower voltage. In another German Patent Document DE 39 25 726 A1 is disclosed the sensing of the excitation current as well as of the excitation voltage and then increasing or reducing the current through the relay winding dependent upon these two values. The known drive circuits, however, are each designed only for operating an individual relay so that the entire circuit must be repeated for each relay given the operation of a number of relays at different times.
Although German Patent Document DE 33 31 678 C2 discloses a circuit arrangement for driving a number of relays which are optionally operable in parallel, a separate drive module is again required. The separate drive module applies the attraction pulses and holding pulse sequences which are generated in a common drive means to the respective relay winding. Here, too, a considerable circuit outlay is involved which is increased with each additional relay used.
The foregoing problem is particularly acute in relays operated in motor vehicles due to the high temperature differences which prevail and the wide fluctuations of battery voltages in such applications.